len<-1e7
val<-rnorm(n = len)
val.names<-rep(c('a','b'),len/2)
val.df<-data.frame(val=val,val.names=val.names)
t1<-system.time(val.df%>%group_by(val.names)%>%summarise(x=mean(val)))
t2<-system.time(aggregate(val~val.names,data=val.df,FUN='mean'))

#缺失值
